1. Field of the Invention
The present invention relates to an image sensor, and more particularly to an apparatus and a method for improving quality of a photographed image.
2. Description of the Related Art
Recently, image sensors are increasingly being developed due to rapid development of imaging appliances. Herein, the term ‘image sensor’ is a general term which refers to devices used when photographing an image by using semiconductors which are sensitive to light. Generally, parts of a target object in the natural world have different luminance and different wavelengths. Accordingly, the image sensors convert light energy (photons) obtained by means of the luminance, the wavelength, etc. of light obtained through a lens into an electrical signal (electrons). Such image sensors representatively include a charge coupled device (“CCD”) image sensor and a complementary metal oxide semiconductor (“CMOS”) image sensor. The two image sensors include a photo detector for generating charges according to intensity of light from the light incident on the image sensors and a circuit for delivering the charges to external devices. Although the above two sensors may generate charges by means of a photo detector through the same procedures, they process the charges differently after generating the charges.
In the CCD image sensor, what transfers charges is an element called “CCD”. The CCD image sensor has been mainly employed for a digital camera or a video camera for a long time.
The CMOS image sensor deals with charges by using switching of a CMOS transistor in place of the CCD. Since the CMOS image sensor has a switch for every photo detector, the CMOS image sensor can directly process charges regardless of places. Accordingly, the CMOS image sensor performs faster reading and consumes lower power as compared with the CCD image sensor. In addition, the CMOS image sensor is made similarly to a fabrication process of a computer chip. Therefore, in the CMOS image sensor, image sensor circuits can be arranged together with image processing circuits.
Such a CMOS image sensor has been mainly employed for portable telephones and small sized digital cameras. Recently, since the CMOS image sensor is employed for high-quality digital cameras, the CMOS image sensor is growing as a challenger of the CCD image sensor.
In the meantime, the CMOS image sensor photographs an image in a cell with a pixel unit by using at least one transistor and at least one photo diode. In other words, the CMOS image sensor is an image capture device which amplifies charges generated due to optical/electric conversion of horizontally-arranged photodiodes by means of internal transistors and reads out an image by one pixel through a switch circuit. Thus, since the CMOS image sensor performs an amplification function for every pixel, the CMOS image sensor can obtain high sensitivity and a high SNR. In addition, the CMOS image sensor has a random access function, so that the CMOS image sensor can read out a predetermined pixel by selecting one horizontal signal line and one vertical signal line.
In addition, since the CMOS image sensor is based on a standard CMOS processing technology, a small-sized chip, a light camera, and on-chip including peripheral driving circuits can be realized, and lower consumption of power can be achieved. Additionally, in the CMOS image sensor, an image can be photographed under weak illumination because current is amplified in proportion to signal charges.
However, a digital camera employing the CMOS image sensor cannot provide the best image quality due to the following causes.
First, since elements used for a pixel array part of the CMOS sensor are sensitive to change of a temperature, dark current occurs depending on a changed temperature. The dark current becomes a cause in which an undesired black level is included in an image.
Second, in the digital camera employing the CMOS image sensor, when a target object such as the fluorescent light having great light intensity is photographed, a ghost image appears due to saturation of an image signal.
Third, if interpolation is achieved with respect to an image processed through gamma correction, the optimum value for gamma correction cannot be obtained correspondingly to a dead pixel restored through the interpolation.